1. Field of the Invention
The present invention relates to a process for the treatment of waste materials, and in particular to a process for the treatment of incinerator residue produced from commercial, industrial and municipal refuse, to an aggregate material produced by that process, and to the use of that aggregate material.
2. Background of the Invention
The problems associated with disposing of waste materials in an environmentally sound manner are well known. One of the serious concerns our consumption-oriented modern industrial society faces is what to do with municipal refuse. Traditional solutions, especially disposal of unprocessed refuse in landfills, are rapidly losing their viability, as acceptable disposal sites become increasingly difficult to find, and as such sites have become recognized as sources of unacceptable pollution of their environs.
While raw municipal solid waste contains many materials that can be separated and recycled, there is always a residuum which must be somehow disposed of. One general approach to municipal waste disposal has been incineration, often now combined with cogeneration of power such as in modern "trash-to-steam" facilities. Municipal refuse incinerators must include means for minimizing the emission of environmentally harmful substances, such as HCl, SO.sub.2, NO.sub.x, polynuclear aromatic compounds, and dioxin. In addition, ash is a major product of municipal trash incinerators which itself must be disposed of in an environmentally acceptable manner. Two types of residue are produced by incineration: low density fly ash which must be removed by filtration, electrostatic precipitation, or a similar technique from the stack exhaust, and bottom "ash," a residue which is too dense to be carried with the hot exhaust gases.
Incineration may concentrate toxic materials present at low levels in the raw refuse so that the ash produced contains environmentally unacceptable proportions of pollutants such as mercury and cadmium. If untreated incinerator ash is disposed of in a landfill, such toxic material may leach out and seriously contaminate groundwater in the vicinity of the landfill. Disposing of incinerator ash by "landfilling" the ash has been a common solution to the disposal problem. Clearly, there is a need for a process for treating incinerator ash to reduce the likelihood that toxic materials present in the ash, such as "heavy metals" including mercury and cadmium, will leach out to contaminate local groundwater in the landfill area.
Many processes have been proposed for the incineration of municipal wastes and treatment of the residues formed thereby. For example, it has been suggested that fly ash produced by incinerating municipal waste be treated with a slightly acid medium to remove soluble materials, that the soluble contaminants removed thereby be treated chemically to precipitate them, and that the precipitate be immobilized as a compact material. At the same time it has been advised that the incinerator itself be operated to produce an inert slag which can be reused.
Hot bottom "ash" from a municipal refuse incinerator may be first treated by quenching with water in a quench tank. The wet quenched bottom ash can be dried, screened to remove bulk metal, and separated into magnetic and nonmagnetic fractions using a magnetic separator. The fly ash can be disposed of separately, such as by use as a supplement for cement, or it can be combined with the residue in the quench tank. The non-magnetic fraction can be used as an aggregate for subbases or base courses (such as for highway construction or the like), as an aggregate for Portland cement concrete and articles formed therefrom such as concrete masonry blocks, or as an aggregate for bituminous concrete. The non-magnetic fraction includes glass, sand, slag, ash and some magnetic materials. The use of this fraction as an aggregate in Portland cement concrete is complicated by a potential alkali-glass reaction which can cause concrete expansion detrimental to the concrete.
The non-magnetic fraction can be further processed. For example, the glass fraction can be separated and used to produce structural materials such as bricks. The waste glass can be ground, mixed with clay and sodium silicate, pelletized and sintered to give expanded glass aggregate pellets useful as aggregate for Portland cement concrete.
Fine particulate waste can be agglomerated by adding water or a binder liquid to the dry waste powder in a disk or barrel pelletizer, the resulting green pellets being subsequently fired in a rotary kiln or traveling grate sintering machine to produce aggregate for concrete. The rotary kiln can produce a lightweight expanded aggregate having a fused surface, having low water absorption and suitable for cast-in-place concrete. The traveling grate sintering machine produces a more porous aggregate which can be used in concrete blocks.
Portland cement has been used to trap a wide variety of toxic wastes in a solid matrix; including arsenic (U.S. Pat. Nos. 4,329,179; 4,142,912; 4,046,674; 3,933,624; 3,804,750), radwaste (U.S. Pat. Nos. 3,988,258; 4,122,028; 4,017,417; 4,504,317; 4,530,723; 4,533,395), fossil fuel ash (U.S. Pat. No. 4,726,710), acidic liquors containing heavy metals, such as from electroplating and metal finishing (U.S. Pat. No. 4,741,776), washing water filter sludge (U.S. Pat. No. 4,304,674), and oxysludge from steel manufacture (U.S. Pat. No. 4,601,832).
Despite the progress which has been made toward addressing the problems posed by the disposal of solid wastes from municipal, commercial and industrial sources, there remains a substantial need for an inexpensive, reliable method for treating incinerated waste, which often contains significant amounts of toxic materials, such as heavy metals, so that such incinerated waste can be disposed of in an environmentally sound manner.
In attempting to minimize the leaching of hazardous materials from waste such as incinerator ash, many approaches have focused, with varying degrees of success, on isolating the hazardous material from its environment, such as by coating it with an impervious barrier of Portland cement, by vitrification, by the use of impervious plastic barriers, and like measures all aimed at reducing the flow of ground water through the material.
While the incineration of waste materials is becoming an increasing popular response to the disposal prob]em, there has already been a substantial capital investment in incineration facilities by municipal authorities and by industry. Thus, there is a need for an incinerated waste treatment process for carrying out this process which build on this past capital investment and which can also be used in future incineration facilities.
There is a growing need for a process providing for the long term disposal of incinerator ash from municipal solid waste. Further, there is a need for a treatment process which can be used in either a batch or continuous mode to treat incinerator residues. Similarly, there is a need for a process which can treat incinerator residues to provide a useful construction material, the size and strength of which can be controlled in the production process.